cmd/cgo: make typedef map traversal order consistent         So that _cgo_gotypes.go will be the same for the same source         code.
cmd/cgo: make typedef map traversal order consistent
        So that _cgo_gotypes.go will be the same for the same source
        code.

R=nigeltao
CC=golang-dev
http://codereview.appspot.com/6357067

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang